This disclosure relates to improvements in resistivity logging systems. A resistivity log is obtained by measuring the resistance at an electrode emitting current flow from a downhole logging tool which current flows into the adjacent formations of the earth. The current flow penetrates into the earth. The depth of penetration is an important factor. Shallow penetration provides the borehole resistivity, or the resistivity observed within a few inches of the borehole. In large part, the borehole resistivity is altered or impacted in some fashion by the presence of the borehole and the drilling mud in the borehole. By contrast, it is also desirable to obtain the resistivity of the earth's formations deep into the formations at some distance from the borehole. This is a measure of the resistivity of the undisturbed formations. As will be understood, these resistivity values are important for different reasons. The goal in resistivity logging is to obtain indications of resistivity which are used to infer values of porosity, permeability, and fluid saturation. It is helpful to have both data close to the borehole and form the deep formations.
Deep formation resistivity measurements are difficult to obtain because the current flow inevitably turns upwardly and does not penetrate deeply into the adjacent formations. The current flows from the emitting electrode supported on the downhole tool into the earth's formations and is returned to a grounding electrode near the wellhead. The grounding electrode defines the return path which is, on the ordinary scale of wells, more or less parallel to the well. This is especially true when logging at depths commonly encountered in most wells. The current flow is primarily parallel to the borehole in returning to the electrode because the current seeks the least resistant path to return to the electrode, and this is typically the shortest path.
The current can be forced to travel further from the tool and deeper into the earth's formation by using a longer emitting electrode. Assume that the electrode is maintained at a typical operating voltage and is made as long as practicality permits. In that event, the current is forced to flow radially outwardly under the influence of the field from the relatively long electrode. This is desirable to get deep formation penetration. On the other hand, deep penetration serially adds resistance to the measured value and therefore obscures the resistivity up close to the wellbore. Close in borehole resistivity measurements are best obtained from a relatively narrow electrode which permits the current flow to bend upwardly in returning to the grounding electrode.
The apparatus and method of this disclosure set forth a means and method whereby resistivity measurements can be obtained from the adjacent borehole and from deep formations. These measurements are obtained while the resistivity logging system is pulled through the wellbore at typical operating velocities. For instance, a logging rate of one hundred feet per minute is typical. This apparatus obtains resistivity measurements from deep formation as well as the borehole while logging at that rate and presents such measurements in a manner enabling the log interpretation to be carried out easily.
Log interpretation is enhanced by presenting the data in a resistogram plot. The deep formation data is located at one side of the chart or graph. The remaining side includes the resistivity from the borehole area, and the intermediate values are located in a more or less linear fashion in the resistogram graph.
With the foregoing in view, the present disclosure presents a resistivity logging system featuring a central electrode of relatively narrow vertical dimensions symmetrically flanked above and below by electrode pairs. They are preferably equal in length arranged on uniform spacing. The number of electrodes above and below the central electrode can be varied but ten is believed to be an adequate number. The electrodes are normally not connected with the central electrode. They are switched on in pairs so that the central electrode is first operated alone and then additional pairs of electrodes are connected in parallel with it. So to speak, a time variant electrode of ever increasing vertical dimension is accomplished. It is swept at a specified rate. The resistivity measurement obtained from the system is synchronized with the effective width of the emitting array of electrodes to thereby correlate the measured resistivity with the sweep, and this yields data of the borehole resistivity and deep formation resistivity with intermediate values therebetween.